SPQRattus

If you've got creative mode available and are willing to use it, there's a command: /we pacify-water which causes all flowing water blocks in a marked area to be replaced by still water. By default it targets fresh water, but you can make it work on salt water by using /we pacify-water saltwater instead. To mark an area, you can either use the /we command (read about it in the .chb) or spawn yourself a magic wand.